espite efforts to avoid exposing pregnant women to smallpox vaccine, 103 have received it in the national program to prepare for a bioterror attack, the government reported yesterday.

Some of the women did not realize they were pregnant when they were vaccinated, and others conceived in the four weeks after receiving the shot, despite strong warnings not to become pregnant in that time.

Six of the women were health workers, 85 were in the military and 12 others were participants in studies of the vaccine.

They are a tiny fraction of all the women of childbearing age who have been vaccinated: more than 62,000 in the military and more than 6,000 civilians.

But the cases are cause for concern, doctors say, because smallpox vaccine can harm the fetus by causing fetal vaccinia, an infection with the virus used in the vaccine. They also illustrate the difficulty of making sure that people who should not be vaccinated avoid it.

Dr. William Schaffner, an infectious disease expert at Vanderbilt University, said: "Despite a program that was elaborately focused on safety, these inadvertent events happen. That is a large number of women. All these women were informed elaborately about the need not to be pregnant. Fortunately, the risk of fetal vaccinia is very low."

But if the vaccination program is expanded as planned to include larger numbers of people outside the health-care professions, even higher rates of pregnant women might wind up being vaccinated inadvertently, Dr. Schaffner said. And that, he added, along with the expense and time required by the vaccination program, is a reason to reconsider whether the program should continue as planned, to a second, much larger phase.

Although fetal vaccinia is extremely rare, it is serious, often killing the fetus or newborn or causing premature birth.

Doctors from the Defense Department and the Centers for Disease Control and Prevention said the condition is so rare that they did not expect to see any cases among the 103 women. The pregnancies will be monitored, and the outcomes recorded in a national registry.

Dr. Jane Seward, chief of viral vaccine preventable diseases at the centers, said, "It's highly likely that these women will deliver healthy babies." She added that some of the women were worried initially, but felt reassured when they learned that the risk was very low.

Two of the six health workers had early miscarriages, but it is impossible to say whether they were related to the vaccine, Dr. Seward said. Miscarriage is extremely common in the general population, occurring in 16 to 31 percent of all pregnancies. It is especially common early in pregnancy.

Only 50 cases of fetal vaccinia have ever been reported worldwide, including three in the United States, in 1924, 1959 and 1968. Those cases were recognized because the infants or fetuses had vaccinia skin sores or scars. One was born prematurely and died, another was miscarried, and a third was born prematurely, but was healthy and developed normally.

Dr. Seward said researchers estimated that in 1947, when 6.5 million New Yorkers were vaccinated to halt an outbreak, as many as 173,000 pregnant women may have been vaccinated, and yet there were no cases of fetal vaccinia.

Dr. Seward and Col. John Grabenstein, head of the military vaccine agency, said that the pregnancy rate was low, but that they hoped to find ways to improve education and screening to make it even lower.